The present invention relates to an optical scanner used in a laser beam printer, a laser facsimile, a digital copier, or the like.
Many of optical scanners used in laser beam printers and the like each include a semiconductor laser as a light source, a first image formation optical system for linearly focusing a light beam from the light source on an optical deflector to compensate for the tilt of a deflection surface of the optical deflector, a polygon mirror as the optical deflector, and a second image formation optical system for forming uniform spots on a surface to be scanned at a constant speed.
A second image formation optical system in a conventional optical scanner includes a plurality of large glass lenses called an xe2x80x9cfxcex8 lensxe2x80x9d and hence, had problems of difficulty in size reduction and of high cost. In order to achieve reductions in size and cost, an optical scanner as described in JP 11-30710 A has been proposed that includes one curved mirror used as a second image formation optical system.
Schematically, it is described that a light beam from the curved mirror is guided directly to an image surface in the above-mentioned optical scanner. In the optical scanner, however, the reflection angle of a light beam reflected by the curved mirror is small and accordingly, it has been necessary to dispose a reflecting mirror between the curved mirror and a photoconductive drum to actually guide a light beam to the photoconductive drum. In addition, since a cross-sectional shape in a sub scanning direction of the curved mirror described in JP 11-30710 A is not a circular arc shape but a quadratic polynomial shape, processing and measurement of the curved mirror are difficult.
JP 11-153764 A discloses an optical scanner including a second image formation optical system composed of one curved mirror alone without requiring a reflecting mirror. However, suitable conditions for guiding a light beam directly to a surface to be scanned have not been clear. In addition, it is necessary to allow a light beam to be incident obliquely on the curved mirror at a large angle therebetween to guide the light beam directly to the surface to be scanned, but this causes a considerable beam aberration at the mirror plane. However, there has been no sufficient countermeasure to such a considerable beam aberration.
In view of the above-mentioned problems, the present invention is intended to provide an optical scanner having excellent optical performance, guiding a light beam from one curved mirror directly to a photoconductive drum without requiring a reflecting mirror, and allowing the curved mirror to have a shape permitting relatively easy processing and measurement.
An optical scanner according to a basic configuration of the present invention includes: a light source unit for emitting a light beam; an optical deflector for deflecting the light beam from the light source unit so as to cause scanning; a first image formation optical system for forming a line image on a deflection surface of the optical deflector; and a second image formation optical system composed of one curved mirror. The first image formation optical system is disposed between the light source unit and the optical deflector. The second image formation optical system is disposed between the optical deflector and a surface to be scanned. A light beam from the first image formation optical system is incident obliquely on a plane that is parallel to a main scanning direction and contains a line normal to the deflection surface of the optical deflector. A light beam from the optical deflector is incident obliquely on a plane (hereinafter referred to as a xe2x80x9cYZ planexe2x80x9d) that is parallel to the main scanning direction and contains a line normal to a vertex of the curved mirror. In addition, a conditional formula of 10 less than xcex8M less than 35 is satisfied, wherein xcex8M indicates an angle in degrees between the YZ plane and a central axis of a light beam traveling toward the curved mirror from the optical deflector.
According to this configuration, the second image formation optical system is composed of one mirror and reflects a light beam at a large reflection angle satisfying a relationship of 10 less than xcex8M. Hence, the degree of freedom in arrangement increases, no reflecting mirror is required, and a light beam can be guided directly to the surface to be scanned. The upper limit of the above-mentioned conditional formula defines a range in which beam aberration can be compensated.
In the above-mentioned configuration, a light beam is incident obliquely on the curved mirror at a large angle satisfying the relationship of 10 less than xcex8M. This causes a large beam aberration at a mirror plane. In order to compensate for this aberration, in the optical scanner of the present invention, a light beam from the first image formation optical system is allowed to be incident obliquely on a plane that is parallel to the main scanning direction and contains the line normal to the deflection surface of the optical deflector, and the tilt angle of the light beam is limited in a range in which the beam aberration can be compensated.
In order to obtain an excellent spot in the above-mentioned configuration, in a cross section in a sub scanning direction, an angle made by a light beam reflected by the curved mirror with respect to an incident light beam from the deflection surface is set to be negative when a direction of an angle made by a reflected light beam reflected by the deflection surface with respect to an incident light beam from the first image formation optical system is regarded as positive. With this configuration, the reflected light beam and the incident light beam are positioned in the positive and negative directions, respectively. Hence, the beam aberration caused by the oblique incidence is compensated and thus an excellent spot can be obtained.
In order to obtain a more excellent spot, in the above-mentioned configuration, the following conditional formula (1) is allowed to be satisfied:
1.6 less than xcex8M/xcex8P+0.98L/(L+D) less than 2.2xe2x80x83xe2x80x83(1),
wherein xcex8P indicates an angle in degrees between the line normal to the deflection surface and the light beam from the first image formation optical system, L a distance from the deflection surface to the vertex of the curved mirror, and D a distance from the curved mirror to the surface to be scanned.
When the positional relationship among the first image formation optical system, the optical deflector, and the second image formation optical system satisfies the above-mentioned conditional formula, the beam aberration caused by the oblique incidence of a light beam can be compensated suitably. The aberration is caused in an oblique direction when the conditional formula is not satisfied.
In order to achieve a higher resolution, it is desirable that the following conditional formulae (2) and (3) are satisfied:
1.86 less than xcex8M/xcex8P+0.98L/(L+D) less than 1.94xe2x80x83xe2x80x83(2);
and
0.48 less than L/(L+D) less than 0.75xe2x80x83xe2x80x83(3).
When the conditional formula (2) is satisfied, the beam aberration caused by the oblique incidence of a light beam can be compensated further suitably. When the conditional formula (3) is satisfied, the beam aberration can be compensated even at a reflection angle satisfying the relationship of 10 less than xcex8M. When the lower limit is not satisfied, the beam aberration occurs. On the contrary, when the upper limit is not satisfied, a beam diameter in the sub scanning direction considerably varies between the scanning center and periphery. In such cases, it is difficult to obtain a high resolution.
In the optical scanner with the above-mentioned basic configuration, the curved mirror has a cross section with a circular arc shape in the sub scanning direction. With this configuration, the curved mirror is allowed to have a shape permitting relatively easy processing and measurement.
It can be considered to allow the curved mirror to have a shape with different radii of main and sub curvature, which permits compensation for the curvature of field, an error in fo, and curvature of a scanning line, and in addition, to allow the curved mirror to have a freely curved plane with skew lines normal to a generator at respective points thereon.
In other words, it is preferable that the curved mirror has a shape for compensating the curvature of the scanning line caused by oblique incidence. In addition, the curved mirror may be asymmetrical with respect to the YZ plane. Furthermore, the curved mirror can have a skew shape in which a line normal to each point other than a vertex of a curved line (hereinafter referred to as a xe2x80x9cgeneratorxe2x80x9d) on which the YZ plane and a curved plane of the curved mirror intersect is not contained in the YZ plane. With such configurations, the optical system is allowed to have a simple configuration, and the curvature of the scanning line can be compensated while the beam aberration caused by oblique incidence of a light beam is compensated.
In the curved mirror with the skew shape, preferably, an angle in degrees between the YZ plane and the line normal to each point on the generator is set to increase with distance from the vertex. Furthermore, in the curved mirror with the skew shape, preferably, a direction of an angle made by the line normal to each point on the generator with respect to the YZ plane is set to be positive when an angle made by a light beam reflected from the curved mirror with respect to an incident light beam from the deflection surface is regarded as positive.
In the optical scanner with the above-mentioned basic configuration, preferably, the curved mirror is an anamorphic mirror with different radii of curvature in the main and sub scanning directions at the vertex. The curved mirror can have a mirror plane with concave shapes both in the main and sub scanning directions. In addition, the curved mirror can have a mirror plane whose refracting power in the sub scanning direction varies between the center and periphery in the main scanning direction. Furthermore, the curved mirror can have a cross section in the sub scanning direction whose radius of curvature does not depend on its cross-sectional shape in the main scanning direction.
In the optical scanner with the above-mentioned basic configuration, the first image formation optical system can be configured so as to convert the light beam from the light source unit into a convergent light beam with respect to the main scanning direction.
The above-mentioned configurations allow excellent performance to be obtained with respect to the curvature of fields in the main and sub scanning directions and the fxcex8 characteristic.
In the optical scanner with the above-mentioned basic configuration, the light source unit also can include a wavelength variable light source and a wavelength controller. According to this configuration, since the size of a spot is substantially proportional to a wavelength to be used, control of the wavelength allows the size of a spot formed on the photoconductive drum to be controlled freely. In addition, since the second image formation optical system is composed of the reflecting mirror alone, no chromatic aberration occurs and thus it is possible to change the resolution freely without deteriorating the other performances such as the fxcex8 characteristic.
An optical scanner according to another basic configuration of the present invention includes: a light source unit for emitting a light beam; an optical deflector for deflecting the light beam from the light source unit so as to cause scanning; a first image formation optical system for forming a line image on a deflection surface of the optical deflector; and a second image formation optical system composed of one curved mirror. The first image formation optical system is disposed between the light source unit and the optical deflector. The second image formation optical system is disposed between the optical deflector and a surface to be scanned. A light beam from the first image formation optical system is incident obliquely on a plane that is parallel to a main scanning direction and contains a line normal to the deflection surface of the optical deflector, and a light beam from the optical deflector is incident obliquely on a plane (hereinafter referred to as a xe2x80x9cYZ planexe2x80x9d) that is parallel to the main scanning direction and contains a line normal to a vertex of the curved mirror. In addition, the light source unit includes at least two light sources, a light mixing member is provided for mixing light beams from the at least two light sources and is disposed between the light source unit and the optical deflector, and a conditional formula of 10 less than xcex8M less than 35 is satisfied, wherein xcex8M indicates an angle in degrees between the YZ plane and a central axis of a light beam traveling toward the curved mirror.
According to this configuration, since at least two light beams can be scanned in one-time scanning, at least twice as much line image data can be scanned as in the case of using one light source on the surface to be scanned. In addition, since the second image formation optical system is composed of one mirror and a light beam is reflected at a large reflection angle satisfying the relationship of 10 less than xcex8M, the degree of freedom in arrangement increases and thus a light beam can be guided directly to the surface to be scanned without requiring a reflecting mirror.
For example, a dichroic mirror can be used as the light mixing member. The dichroic mirror reflects and transmits light selectively through wavelength selection and thus can mix lights efficiently. Furthermore, a half mirror also can be used. The half mirror can be processed easily and thus can mix lights at low cost.
The optical scanner with the above-mentioned configuration can be provided with a light separating member for separating a light beam present between the optical deflector and the surface to be scanned. When the light separating member for separating a light beam is disposed between the optical deflector and the surface to be scanned, at least two line images can be formed simultaneously on the surface to be scanned by one-time scanning. Accordingly, an effect can be obtained that a rate of image formation or image reading can be at least doubled.
It is preferable that for instance, a diffraction grating or a dichroic mirror is used as the light separating member. With the diffraction grating, an incident light beam is diffracted at a diffraction angle varying depending on the wavelength, and thus light can be separated efficiently at low cost. With the dichroic mirror, it reflects or transmits light through wavelength selection, and thus light can be separated efficiently.
In the optical scanner with the above-mentioned configuration, it also is possible to allow the light beams emitted from the at least two light sources included in the light source unit to have different wavelengths. Even if lights with different wavelengths are used, chromatic aberration, which occurs generally, does not occur at all, since the second image formation optical system is composed of the reflecting mirror alone. Consequently, it is possible to achieve color image formation or color image reading with a high resolution.
When an image reader or an image forming apparatus is configured using an optical scanner with any one of the above-mentioned configurations, a small high-speed and high-resolution image reader or image forming apparatus can be obtained at low cost.
A color image forming apparatus includes a conveying member, a transfer member, and an optical scanner. In the conveying member, a plurality of image forming units corresponding to a plurality of colors are assembled in a substantially cylindrical form. Each of the image forming units includes a developing unit and a photoconductor. The respective image forming units are maintained to be arranged circumferentially in the substantially cylindrical form. The conveying member simultaneously rotates the plurality of image forming units around a central axis of the substantially cylindrical form to move each of the respective image forming units between an image formation position and a stand-by position. The transfer member has a transfer body that can come into contact with the photoconductor of an image forming unit positioned at the image formation position, sequentially transfers toner images of respective colors formed on the respective photoconductors onto the transfer body as the image forming unit at the image formation position is changed successively, and superimposes the toner images of the respective colors to form a color toner image on the transfer body. The optical scanner has any one of the above-mentioned configurations and exposes the photoconductor of each image forming unit to light.
According to this configuration, optimization of the arrangement of the curved mirror and the angle of reflection of a light beam allows a small color image forming apparatus to be obtained at low cost.
In the color image forming apparatus with the above-mentioned configuration, the curved mirror of the second image formation optical system in the optical scanner can be disposed in the vicinity of the central axis of the cylindrical form.
Moreover, in the color image forming apparatus with the above-mentioned configuration, it is preferable that the optical scanner is configured to achieve the following: a light beam from the first image formation optical system is incident obliquely on a plane that is parallel to the main scanning direction and contains a line normal to the deflection surface of the optical deflector, a light beam from the optical deflector is incident obliquely on a plane (a YZ plane) that is parallel to the main scanning direction and contains a line normal to the vertex of the curved mirror, and a conditional formula of 12.5 less than xcex8M less than 17.5 is satisfied, wherein xcex8M indicates an angle in degrees between the YZ plane and a central axis of a light beam traveling toward the curved mirror.